The invention relates to mouldings made from biological residual material, such as potato skins.
For numerous applications, such as profiles, pipes, channels, pots and other containers, there is a need for biodegradable mouldings that can be produced from inexpensive materials. For applications of this type it has been proposed to process starch, if appropriate mixed with other polymers and fillers, and to shape this, for example by means of extrusion. Examples of this are described in WO 92/02559 and EP-A-707034. However, products of this type have a number of disadvantages, including high production costs and high processing temperatures.
Surprisingly it has now been found that potato skins can be processed in a relatively simple and inexpensive process to give robust, but nevertheless readily degradable mouldings. The process according to the invention therefore encompasses the processing of potato skins or similar material by the application of thermomechanical treatment and shaping. It is assumed that the combination of starch, cellulose, pectin, cork and inorganic materials is responsible for the advantageous mechanical properties and the relatively high resistance to water of the mouldings, for example in comparison with the properties of material based on pulp.
The starting material used is potato skins, that is to say the residual material produced in any process for peeling potatoes. The most common method of peeling is steam peeling, in which potatoes are heated for approximately 30 seconds with steam under elevated pressure, after which the skins are removed from the potatoes using a water jet. The skin material thus obtained does not require further pretreatment; at most any residues of sand or soil are removed if necessary. Skins obtained by scraping, shaving and other mechanical processes can also be used. Residual material from other crops, such as cassava skins and chaff from wheat or other grain, can be used as the starting material instead of or in combination with potato skins.
In addition to the residual material, other biodegradable material such as starch, cellulose (derivatives), guar gum, carob flour, tragacanth, pectin, gum arabic or other gums, natural rubber, polyesters such as polycaprolactone and polylactic acid, proteins such as gluten and casein can also be used. In particular the co-use of protein hydrolysis products and biodegradable polyesters is advantageous because the dimensional stability and the waterproof characteristics of the final moulding can be increased in this way. Examples of proteins and protein hydrolysis products are hydrolysed keratin, gluten and zein. Examples of water-resistant degradable polyesters are polylactic acid and polycaprolactone. Polylactic acid has the additional advantage that it is available as an inexpensive material as a result of acidification of the skin material. The lactic acid thus formed can be separated off therefrom, for example in vapour form, after which it is polymerised in the conventional manner and the polymer is added back to the skin material. The quantity of material such as protein or polyester which is added can vary from a few percent up to, for example, 60%, based on the dry skin material. Especially in the case of larger quantities (for example 20-60%) the added material can serve as continuous phase. The material added can be added before, during or after the thermomechanical treatment and before the shaping treatment. If the material is added after the thermomechanical treatment, a mixing step, for example a kneading step, must then be employed. An added protein (hydrolysis product) or polysaccharide can, if necessary, be partially crosslinked during the thermomechanical treatment so that a polymer network is produced. The customary crosslinking agents, such as dialdehydes, diamines, epi-chlorohydrin and the like can be used for crosslinking, in amounts of, for example, 0.1-10% by wt with respect to the crosslinkable material.
The skin material can also be chemically slightly modified, for example by oxidation.
The skins can be comminuted prior to the thermomechanical treatment. Said comminution can be effected in any conventional manner, for example by grinding, sieving, pureeing or scraping. A plasticiser such as a polyol (glycol, diethylene glycol or another alkylene glycol or polyalkylene glycol, glycerol, glycerol monoester and the like), citric acid ester or urea is then preferably added to the skins, although water alone can also suffice. The quantity of water is preferably 3-35% by wt with respect to the total amount of degradable polymer. The quantity of additional plasticiser, such as glycerol, is preferably 0-25% by wt. An emulsifier or flow improver such as lecithin or a monoglyceride (for example 0.5-5% by wt), a solvent such as an oil (for example castor oil), fatty acid or metal salt thereof (for example calcium stearate) can also be added. To increase the strength and/or volume of the mouldings a filler such as a natural fiber, for example flax, straw, elephant grass, cotton, jute or paper, can be added, for example in an amount of 5-50% by wt with respect to the skin material. Inorganic fillers such as lime or chalk can also be added. The addition of lime can further increase the water resistance of the end product by lowering the solubility of pectin. Other possible additives are colorants, preservatives and in particular swelling or blowing agents such as sodium bicarbonate and nucleating substances such as talc.
The mixture of comminuted skins and additives is then subjected to a thermomechanical treatment. During this operation it can be brought into granule/pellet form in accordance with granulating or pelletising processes known per se, for example by extrusion in a twin screw extruder at elevated temperature (60-180xc2x0 C., in particular 100-150xc2x0 C.). The size of the granules is determined by the rate of granulation and any grinding steps following granulation. Depending on the presence of other plasticisers, the water content during extrusion can vary from 3 to 35% by wt.
The product obtained can be conditioned to a moisture content optimum for the process. Depending on the content of other plasticisers, the moisture content after conditioning varies between 4 and 30%. The conditioned product can then be introduced into a mould having the shape of the product finally to be produced. If the conditioned product is in the form of granules, a quantity of adhesive (for example a mixture of a native starch and glycerol) can be added in order to promote the adhesion between the granules during shaping.
Shaping can be effected by casting, injection moulding, pressing and similar techniques. If voluminous products arc desired, shaping can be effected by foaming with the aid of a blowing agent, such as carbon dioxide, lower alkanes or, in particular, water. A suitable method of foaming is, for example, foaming making use of electromagnetic radiation, in particular that in the microwave range (frequency between 20 MHz and 10 GHz and in particular between 50 MHz and 5 GHz). For this operation use is made of the absorption of radiant energy by water, glycerol or other dipolar substance present in the product, said substance heating up within a very short time and being converted into the vapour form. During this operation two processes proceed simultaneously: firstly foaming as a consequence of evaporation of water or the other dipole-containing substance. The advantage here is that no thermal energy has to be supplied from outside. Secondly, the loose material will simultaneously be xe2x80x9cweldedxe2x80x9d together to form a three-dimensional foam moulding. For this purpose it is important that the outside of the granules is meltable, for which an at least partially thermoplastic behaviour of the treated skin material is required. Such a foaming process must take place rapidly, that is to say within a few seconds. This can be achieved by using a microwave source of high power (up to, for example, 50 kW) or by using a combination of microwave generator and mould in which the pressure can be varied rapidly. After the material has been brought into the desired foam shape in this way the mould is opened and the product is removed.
The products obtained using the process according to the invention can have any desired shape. An important application is that of continuous mouldings, such as profiles, tubes, channels, sheets and the like. Such mouldings can be porous, for example when used as a covering for an article which is to swell in an aqueous medium. Discontinuous articles, such as pots, boxes and the like, can also be produced. One example thereof is packaging, for example for fragile equipment, glassware, articles for posting and the like. The product can also be in the form of spheres, discs and the like which can be used as fill material in packages. Advantages of this material are its natural antistatic properties, the fact that water-soluble variants are possible and the biodegradability and compostability. In the case of foamed material the low density is also an advantage for many applications.
Depending on the desired application, it can be useful to apply a protective coating to the moulding obtained. Such a coating can be, for example, a wax coating or a polymer coating. Polymers which can be used for this purpose are thermoplastic or thermosetting polymers, depending on the treatment and the application, such as a natural rubber or a polyester, preferably a degradable polyester such as a polylactic acid or poly-xcex5-caprolactone.